Nonhuman primates of the species Erythrocebus patas (patas), Macaca fascicularis (cynomologus) and Cebus apella (cebus), are subjected to direct-acting or metabolism-dependent chemical carcinogens by transplacental or direct exposure. In some cases the carcinogen-treated animals are subsequently exposed to chemicals that promote the development of neoplasms in rodents. Mechanisms of organ and species differences in the effects of chemical carcinogens and tumor promoters among rodent and nonhuman primate species are investigated. Induced tumors are evaluated by light microscopy using standard staining procedures, histochemical techniques and electron microscopy and are assayed for in vitro cultivability and transplantability to rodents. Selected tumors are subjected to DNA extraction and attempts are made to transfect NIH 3T3 cells with their DNA. These studies have shown that intrinsic susceptibility to transplacental carcinogenesis is greatest in nonhuman primates early in gestation and have provided the only animal model of chemically inducible gestational choriocarcinoma.